This invention relates to soldering processes; and more particularly, it relates to processes for soldering component leads to I/O (input/output) pads on printed circuit boards which are populated with other electrical components that are already soldered to the board.
In the prior art, one conventional process by which components were soldered to a circuit board included the steps of--providing the board with via holes which corresponded in position and number to all of the I/O leads on the components which were to be soldered to the board, placing all of the components on one side of the board such that their leads went through the via holes, and passing the opposite side of the board through a solder wave. This method, however, is not suitable for fabricating more fully populated printed circuit boards in which components are soldered to both of the boards' surfaces.
Other processes which are called "infrared reflow" or "vapor phase reflow" are used to solder components to both sides of a circuit board. With either of these processes, the leads of the various components or the I/O pads on the boards to which the leads are to be attached are provided with solder paste; all of the components are positioned on the board and held in place mechanically; and the entire assembly is placed in an infrared oven or gas vapor. There, the solder paste melts; and it subsequently hardens after the assembly is removed from the oven or gas vapor.
However, in certain situations it is desirable to not solder all of the components to the board at the same time. For example, if some of the components are very expensive (such as I/O connectors which may cost several hundred dollars), it may be desirable to put those components on the board last. That enables the remainder of the board to the fabricated and tested. Then, if the test detects a nonrepairable fault, the partially populated board can be scrapped without wasting any of the expensive components.
To partially populate a board with circuit components, solder them place, and thereafter fully populate the board and resolder it, infrared reflow and vapor phase reflow can be used. However, when the board is fully populated and put in the infrared oven or gas vapor, the solder in the joints of all of the components which were previously soldered to the board will melt for a second time. When a soldered joint is reflowed, some of the solder will migrate away from the joint; and if this migration becomes too large, an open circuit of the joint can result. Thus, reflowing previously soldered joints will result in reliability problems.
One way to complete the fabrication of a partially populated board would be to solder the remaining components by hand. However, that method being labor intensive, is expensive, slow, and subject to human error. Further, for certain types of components, the terminals that are to be soldered are partially hidden from view.
One other prior art method of completing the board's fabrication would be to provide respective nozzles that have the same shape as the components which remain to be soldered, sequentially place each nozzle over its corresponding component, and force hot gas through the nozzle when it is in place. However, such a method is undesirable because differently shaped nozzles are needed for each differently shaped component; and stocking and changing the nozzles for each component shape is both time-consuming and expensive. Further, the above method requires some mechanism by which the circuit board or nozzle is manually moved under a microscope in seven different directions (i.e., left and right, backward and forward, up and down, and also rotation) in order to align the nozzle over a component on a circuit board, and getting a precise alignment in all seven directions is very tedious. Also, the hot gas from a nozzle which has the same shape as a circuit component and which is placed over that component will heat up the component itself. And this heat can damage the component, or it can destroy certain seals such as a lid seal, or it can melt various adhesives such as a heat sink attach adhesive.
Accordingly, a primary object of the invention is to provide a soldering method in which all of the above problems are avoided.